Neutrinos give US best shot at post-Tevatron glory

THE world’s science superpower may no longer possess the world’s most powerful particle smasher, but it isn’t giving up the ghost.

Last week, in the wake of September’s shutdown of the Tevatron particle accelerator at Fermilab in Batavia, Illinois, 500 US physicists met at a workshop in Rockville, Maryland. They were there to discuss their prospects for making major breakthroughs in the context of a government that is increasingly strapped for cash, and without the mighty Tevatron, which was shut down because it could not compete with the Large Hadron Collider at CERN, near Geneva in Switzerland.

The initiatives discussed at the workshop could reshape the pace and type of discoveries that are made about our universe. “The enthusiasm and the energy at the workshop is palpable,” said Persis Drell, director of the SLAC National Accelerator Laboratory in Stanford, California, speaking from the meeting on 2 December.

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On the upside are US experiments that are poised to shed light on several of the most burning questions in physics, such as whether neutrinos really do break the cosmic speed limit and the identity of dark matter, the stubbornly mysterious substance that makes up 80 per cent of the universe’s matter.

On the downside, the most ambitious physics experiment currently under consideration in the US hangs in the balance due to tight budget constraints. The Long Baseline Neutrino Experiment (LBNE) would be the biggest ever to measure a beam of neutrinos and could solve a mystery at the heart of antimatter, but it may prove too costly for a frugal Congress.

Yet the first big post-Tevatron result to come out of the US might still be of the neutrino variety. In September, the OPERA experiment in Gran Sasso, Italy, announced that neutrinos, ghostly subatomic particles, had seemingly travelled faster than light in apparent violation of a bedrock of modern physics, Einstein’s theory of special relativity. This electrified the physics community and the wider world. And the timing was serendipitous for the US&colon; the MINOS experiment, which already sends neutrinos from Fermilab to the Soudan mine (pictured, left) in Minnesota, 800 kilometres away, should be able to confirm or refute the claim.

Researchers are currently upgrading the equipment at both sites to enable better measurement of the neutrino travel time. MINOS may be able to make some speed measurements before March 2012, when the proton beam used to make the neutrinos at Fermilab is scheduled to be shut down for a year, says Fermilab director Pier Oddone. More thorough measurements will follow in 2013 and an analysis in 2014.

Meanwhile, another neutrino experiment, the Enriched Xenon Observatory, is already collecting data in a cavern in New Mexico. It is attempting to observe rare decays of xenon nuclei, which occur with the help of neutrinos. Since the rate of these decays depends on how massive neutrinos are, this would be a roundabout way of measuring neutrino masses, which has never been done. So far, only upper limits on the neutrino mass exist.

It’s not all good news for neutrinos, however, thanks to the uncertainty around the planned LBNE, which could by 2020 settle a mystery at the heart of our understanding of antimatter.

No one knows why matter and antimatter in the early universe failed to annihilate each other completely, and instead left an excess of matter to produce everything we see in the universe today. Some theorists have proposed neutrinos as the source of this asymmetry. If they are, neutrinos and antineutrinos must behave slightly differently, which could show up as differences in the rate of flip-flopping between the three types of neutrino.

This phenomenon, whereby neutrinos spontaneously flip between types as they zip along, has been measured by other detectors, but LBNE would do it precisely enough to attack the antimatter problem more powerfully than before. The trouble is that its two design options are a 200,000-tonne underground water tank built by expanding the Homestake mine in South Dakota to form an enormous cavern 100 metres tall and 65 metres wide, or a smaller but still massive detector using 34,000 tonnes of liquid argon. Neither will come cheap.

The world’s largest neutrino detector could attack the antimatter problem for the first time

Last year, the US National Science Foundation declined to fund LBNE. The Department of Energy might pay, but with Congress looking for cuts, the outlook is precarious. Other countries may still build detectors that can make similar measurements, but these efforts are either not as far along or will be less powerful in some respects (see “Antimatter goes international”).

Much stronger are US prospects for identifying dark matter, thanks to a relatively cheap experiment called the Large Underground Xenon detector (pictured, right), which will also be based at Homestake.

LUX uses liquid xenon as a target for dark-matter particles to hit and thereby reveal themselves. Its detectors have already been built and are being tested above ground prior to being installed in 2012. LUX will be about 10 times as sensitive as previous searches, so could be the first to see these elusive particles.

The bottom line is that if the US plays its cards right, it can remain a big player in particle physics without the Tevatron. “We are at a critical point and have to make a decision whether to stay in the game in a significant way,” says Oddone. “We have the opportunity to have such a powerful programme that the world will come to work with us.” But the US can’t take this for granted. “I don’t think this will sit [still] forever. If we don’t move, other countries will.”

Antimatter goes international

A planned neutrino detector in a US mine – the Long Baseline Neutrino Experiment – could unlock mysteries about antimatter, but its future is uncertain (see main story). Can other nations come to antimatter’s rescue?

The European Union has plans to build a similar experiment, LAGUNA, but they are very preliminary. For example, no site has been chosen.

Meanwhile, a proposed Japanese experiment, Hyper-Kamiokande, would have a shorter distance between the neutrino source and detector than LBNE, making some aspects of the measurement harder (arxiv.org/abs/1109.3262).

If the US commits to funding LBNE now, then Europe and Japan may collaborate on LBNE rather than developing competing experiments. “My expectation is if we go ahead, other countries will help us,” says Pier Oddone of Fermilab in Batavia, Illinois. “But nobody’s going to come and tie their fate to ours if the perception is that the US can’t decide what it wants to do.”